Ran fine until I set for Heat at 58, Cool at 78 and left for ~1 month. It would not have come on for the whole month. Now (as we are trying to pass inspection to close on sale) the compressor/outside fan kick on, but the air handler fan will not come on. This happens in Heat or Cool. However, if I set to EH, the fan starts with the heat strips (maybe a safety thing?). This is true whether fan control is in Fan or Auto. Even if Heat and Cool is Off, Fan should run if set to Fan, right? No joy. Only way it runs at all is to turn to Emergency Heat.

Can it be anything but the thermostat?

The fan runs fine in Emergency heat. If it was Float switch, it would kill the outside unit too, I would think.

What should I check, or do I just get a new thermostat and hope that's it?

Not a float switch issue.
Yes.... the heat sequencer turns the fan on.

This is a somewhat complicated and expensive system.

I came back and changed my reply. That thermostat is a communicating thermostat. It's doubtful it's bad. It sends data commands to an air handler mounted control or EIM (equipment interface module).

Thst EIM can be seen on page 15 of the installation manual. You can open the cover to that module and try a jumper between Rc and G. If that brings the blower on.... the problem is the EIM or remotely a thermostat problem. My guess is nothing will happen and you may need to call a tech in to troubleshoot the fan control board in the air handler.

Thanks to all for your help. I had gotten a little farther. I checked float switch anyway, and it was good. Checked at stat and had voltage. Once I got into the air handler, tested all the thermostat control wires for continuity to be sure there were no breaks in signal - all conductors were fine (I had replaced the attic ladder at the same time as the failure, and the thermostat wire was only about a foot away, so I thought there was a possibility I cut it or screwed through it, but it was fine). So I was headed to the control board to see what I could find.

To my chagrin, it had a sticker saying there is no longer a control board in this unit, and all control electronics are built into the blower motor. Ugh!

So I called a pro - the original installer. Learned I had a full parts warranty, but I would have to cover labor. They quoted $354 to swap the motor. However, when we got back to unit, it RAN - perfectly. Of course.

I had taken all the wire nuts loose (one at a time) as I checked the wire back to the thermostat terminal. It is possible there was a loose connection in one of the wire nuts, and I corrected the issue. Otherwise, it is a blower motor that is flaky and it will happen again. Since we are in the middle of selling the house, we are going to wait and see. If it fails again during the time between now and closing, we'll get the motor replaced under warranty (for the $354 labor charge). If not, the warranty transfers to the new owner for the next 22 months, so at least they are covered for the majority of the cost.

Still, not sure if I fixed it by 'finding' a loose connection causing an open, or if the motor is flaky and failing. Sad thing is I may never know.